Electrochemical cells with lithium negative electrodes

ABSTRACT

Electrochemical cells utilizing lithium negative electrodes capable of discharge at high rate in which the lithium electrode operates in substantially insoluble manner utilizing a nonaqueous electrolyte in which lithium and its oxidation products formed during discharge are insoluble, in which the lithium is not corroded and which has acceptable conductivity and in which excessive electrode polarization and passivation is prevented. The composition of the non-aqueous electrolyte comprises at least a solution of a tetrabutylammonium salt in propylene carbonate and advantageously a solution of tetrabutylammonium chloride (C4H9)4 NCl in propylene carbonate mixed with tetrabutylammonium perchlorate. The concentration of tetrabutylammonium salts in propylene carbonate is preferably in the range of 0.5 to 1 mole per liter and advantageously the ratio of tetrabutylammonium perchlorate concentration to tetrabutylammonium chloride concentration expressed in moles per liter does not exceed 10.

United States Patent Caiola et al.

[451 Apr. 25, 1972 Robert Guy, both of Grenoble; Jean- Claude Sohm,Meylan, all of France 221 Filed: Oct.2, 1969 21 Appl.No.: 863,202

[73] Assignee:

[30] Foreign Application Priority Data Oct. 4, 1968 France ..l68904 [52]U.S. Cl ..l36/6, 136/155 [51] Int. Cl. ..H0lm 43/06 [58] Field of Search..l36/6, 154, 155

[56] References Cited UNITED STATES PATENTS 3,567,515 3/1971 Maricle eta1 ..136/154 3,468,716 9/1969 Eisenberg ..136/154X FOREIGN PATENTS ORAPPLICATIONS 1,100,163 H1968 GreatBritain ..l36/155 1,541,885 10/1968France ..l36/154 Primary Examiner-Donald L. Walton Attorney-Kenyon &Kenyon Reilly Carr & Chapin [5 7] ABSTRACT Electrochemical cellsutilizing. lithium negative electrodes capable of discharge at high ratein which the lithium electrode operates in substantially insolublemanner utilizing a non-aqueous electrolyte in which lithium and itsoxidation products formed during discharge are insoluble, in which thelithium is not corroded and which has acceptable conductivity and inwhich excessive electrode polarization and passivation is prevented. Thecomposition of the non-aqueous electrolyte comprises at least a solutionof a tetrabutylammonium salt in propylene carbonate and advantageously asolution of tetrabutylammonium chloride (C 11 NC] in propylene carbonatemixed with tetrabutylammonium perchlorate. The concentration oftetrabutylammonium salts in propylene carbonate is preferably in therange of 0.5 to 1 mole per liter and advantageously the ratio oftetrabutylammonium perchlorate concentration to tetrabutylammoniumchloride concentration expressed in moles per liter does not exceed 10.

7 Claims, 5 Drawing Figures ELECTROCHEMICAL CELLS WITH LITHIUM NEGATIVEELECTRODES RELATED APPLICATIONS Related applications, Ser. Nos. 643,946,now U.S. Pat. No. 3,542,601; 693,320, now U.S. Pat. No. 3,542,602;695,311, now U.S. Pat. No. 3,533,853; 718,974, now U.S. Pat. No.3,511,716 and 773,015, now U.S. Pat. No. 3,540,938 filed respectively onor about June 6, 1967; Dec. 23, 1967; Dec. 26, 1967; Mar. 21, 1968; andNov. 4, 1968 are co-pending.

BRIEF SUMMARY OF INVENTION Lithium electrodes are of great interestsince lithium is an electronegative metal on the one hand and has a lowatomic weight on the other hand. Lithium used as a negative electrode inan electrochemical cell is therefore able to yield high power per unitof volume or mass.

Lithium has the further advantage of a high reaction rate in the oxidedlithium-reduced lithium system and a relative inertia to the traces ofwater which may be contained in organic solvents.

When lithium is used as negative electrode in an electrochemical cell,due to its strong electro-negativity, it is necessary to useelectrolytes of the non-aqueous type which do not corrode lithium. Theoriginal aim has been to provide primary cells in which the lithiumelectrode is able to be discharged at a very high rate. In this case,the negative electrode advantageously operates by dissolving duringdischarge so that its polarization is prevented.

The same view has been taken of a soluble negative lithium electrodeduring the subsequent development of secondary cells utilizing arechargeable lithium electrode.

When utilizing an electrode of the soluble type in a secondary orrechargeable cell, during discharge, the active metal of the electrodeis oxided and dissolved in the electrolyte in its oxided state, isreduced and electroplated on the conductive carrier of the negativeelectrode during charging.

However, with such a type of electrode, during discharge, theelectrolytic deposits of the metal constituting the active material ofthe negative electrode are not obtained in a coherent and uniform shape.Needles and trees tend to form in such deposits. Such needles and trees,on growing, may build conductive bridges between two electrodes and thuscause short-circuiting. The cell can then definitely fail and in allevents its life is much shortened.

In order to prevent such short-comings, and object of this invention isto provide a secondary cell in which the lithium electrode operates in asubstantially insoluble way. However, the operation of such electrodesof the insoluble type at rates sufficiently high to be of any practicaluse, gives rise to a number of problems, one of the most important ofwhich being prevention of the polarization or passivation of theinsoluble lithium electrode. First, a satisfactory electrolyte must befound, i.e., one in which lithium and its oxidation products formedduring discharge are insoluble, in which lithium is not corroded, whichhas a suitable conductivity and in which the electrodes are not too muchpolarized and passivated during the cell operation.

In short, the desired electrolyte must be sufficiently conductive, mustnot react with lithium, and must not dissolve the product of theoxidation reaction of lithium during discharge. Further, the negativelithium electrode must supply high rate discharges without anysubstantial polarization.

This invention comprises associating a negative lithium electrode and anelectrolyte having a particular composition so that this electrodeoperates as a substantially insoluble electrode, and is able to yieldsufficiently high rate discharges.

According to one feature of the invention, the electrolyte associatedwith the negative lithium electrode comprises at least a solution of atetrabutylammonium salt in propylene carbonate, and advantageously asolution of tetrabutylammonium chloride (Cd-I NCl in propylenecarbonate.

According to another feature of the invention, the elec trolyticoxidation of lithium being caused by the chloride ions,

the negative electrode uses the oxidation reduction system LiCl/Li.

According to still another feature of the invention tetrabutylammoniumperchlorate (Cd-1 NCIO, is added to the electrolyte and this increasesthe permissible maximum density of .eharging current withoutpolarization.

According to a preferred embodiment of the invention, the concentrationof tetrabutylammonium salts in propylene carbonate is in the 0.5 to 1mole per liter range.

Advantageously, the ratio of tetrabutylammonium perchlorateconcentration to tetrabutylammonium chloride concentration expressed inmoles per liter does not exceed 10.

The invention will be more clearly understood with reference to thefollowing detailed description, giving some experimental data andillustrated in the accompanying drawings, given merely as examples, andin which:

FIG. 1 shows a curve illustrating the dependence of the conductivity ofa solution of tetrabutylammonium chloride in propylene carbonaterelative to its concentration at ambient temperature, the conductivitybeing expressed in mho/cmx 10' and being plotted as ordinates and themole concentration in moles per liter of the solution being plotted asabscissae;

FIG. 2 shows curves defining the electro-active domain ofpropylenecarbonate containing NBut,Cl 0.5 M/l. Curve A corresponding toa platinum electrode and curve B to a lithium electrode, and in thisfigure, the current densities in mA/cm have been plotted as ordinatesand the oxidation reduction potentials in mV referred to the AgCl/Agelectrode have been plotted as abscissae;

FIG. 3 is a cross-sectional view of a lithium electrode used in a cellaccording to the invention;

FIG. 4 shows polarization curves of an insoluble lithium electrodeLi/LiCl immersed in an electrolyte constituted by a solution inpropylene carbonate of:

For Curve D: NBut.,Cl 0.5 MI];

For Curve E: NBut Cl 0.2M/l NBut, ClO, 0.9 M/l For Curve F: NBut Cl 0.1M/l NBuL, C10 0.7 M/l For Curve G: NBut Cl 0.1 M/l NBut, C10 1 M/lrespectively;

FIG. 5 shows oxidation curves at constant current intensity of a lithiumelectrode immersed in an electrolyte constituted by a NBut Cl 0.2 M/lNBut.,ClO, 0.9 M/l solution in propylene carbonate, curves H, l and Jcorresponding to dischargerates of 5.7 and 10 mA/cm respectively. Inthis figure, the discharge times expressed in minutes have been plottedas abscissae and the lithium electrode voltages referred to the AgCl/Agelectrode expressed in millivolts have been plotted as ordinates.

DETAILED DESCRIPTION Solubility tests of salts ionizable in organicsolvents in which lithium is not corroded and stability of lithium insuch electrolytes.

Some polar organic solvents are reported to be inert towards lithium.Solvents put in contact with lithium at ambient temperature for a periodof 5 months are shown in Table I. A slight continuous bubbling has beenobserved with respect to dimethylsulfoxide, but no reaction has beenfound to occur between lithium and the four other solvents.

TABLE I Continued [Physical properties of solvents compatible withlithium] Dielec Melting Boiling tric Structural point point Specificcon- .iolvents formula 0.) C.) gravity stant Butyro- CHz-CH: 42 206 1.1239 lactone I Tetrultydro- CHr-CH: 65 64 0.888 7.4

iurau CH2 CH:

Dlothyl O=S=O 24. 208 1. 777 29 sulfate OCzHsO CzHu The solubilities oflithium chloride, tetramethylammonium and tetrabutylammonium chloridesin these solvents also have beendetermined: the results are recorded inTable II below.

The only solvents suitable for a LiCl/Li electrode operating ininsoluble conditions are those in which LiCl is insoluble and in which atetralkylammonium salt is soluble. Thus, as seen from Table II, amongthe tested materials only propylene carbonateand diethyl sulfate withNBut Cl are feasible. However, the diethyl sulfate +NBut,Cl+Li mixturehas been found to react after a few days and the entireelectrolyte gelsinto a whitish mass. Therefore, propylene carbonate is the onlyremaining suitable solvent. Another tested solvent constituted by amixture of ethylene carbonate and propylene carbonate did not givesatisfactory results, lithium chloride being soluble in this solvent.

In further testing, a piece of lithium was then immersed in propylenecarbonate containing 0.5 mol/liter tetrabutylammonium chloride for aperiod of 5 months; no reaction was found to occur. It should be notedthat tetrabutylammonium chloride has been chosen from among the othertetrabutylammonium salts because it is easily prepared.

The electromotive force of a cell made of two reversible electrodes ofthe insoluble types, namely, LiCl/Li and Ag- ClAg, immersed in the sameelectrolyte containing the Clion can be derived from the variation offree enthalpy during the following reaction:

Li-t-AgCl LiCI+Ag AG 65.5 Kcal E 2.83 volts This value does not dependfrom the selected solvent since the above reaction does not involve anydissolved species. Therefore, if the insoluble electrode LiCl/Lioperates in a reversible manner, its potential referred to the AgCl/Agelectrode should be about 2.83 volts. I

Tetrabutylammonium chloride is very soluble in propylene carbonate; a 3moles per liter solution is obviously not saturated. FIG. 1 shows thevariation of the solution conductivity asa function of concentration. Atambient temperature, the maximum conductivity (5.6X 103 0 "cm) isobtained for a 0.6 moles/liter concentration. Thereafter, conductivityvery steeply decreases as concentration increases: this behavior israther difierent from that of aqueous solutions.

Therefore, it appears advantageous to use solutions ranging from 0.5 Mto 1.0 M/l tetrabutylammonium chloride in propylene carbonate so thatthe cell has the minimum electric resistance under the usual ambienttemperature conditions of operation.

In order to determine the effect of possible water traces, tests weremade with 97 percent propylene carbonate. Water was removed from the 3percent impurities after dissolution of NBut Cl by stirring over calciumhydride for a period of about 15 hours. 50 to ppm of water then remainin solution. Calcium hydroxide formed by the reaction and the excess ofhydride are removed by centrifugation. Experience has shown that afurther dehydration was not necessary since when lithium reduces thelast traces of water, LiCl is mainly formed. This is due to the factthat the Clion is present in the solution whereas no OI-Iis present; thesolubility coefficient of LiCl is therefore reached before that of LiOH, though lithium hydroxide is less soluble than lithium chloride inpropylene carbonate. More accurate measurements have shown that thesolubility of LiCl in propylene carbonate is 3 X 10" mole/liter. Lithiumchloride which is the discharge product of lithium, may, therefore, beconsidered as substantially insoluble in the electrolyte developedaccording to the invention.

In order to study the electroactivity domain of propylene carbonatecontaining NBut Cl, a I-I-shaped cell has been used, its twocompartments being separated by sintered glass (porosity n3) to preventmixing oxidation and reaction products of the electrolyte. As may beseen from FIG. 2, the electrolyte is reduced at substantially morenegative potentials on the lithium electrode, viz. 3,400 mV (Curve A),than on the platinum electrode, viz. 2,600 mV (curve B). Taking intoaccount the fact that the oxidation reduction potential of the LiCl/Lisystem is about -2,800 mV, it may be seen that during cathodepolarization of the lithium electrode, therefore, during charging,lithium chloride will be reduced first, the electrolyte being reduced onlithium only after the completion of the first reduction, i. e. that ofthe active material.

This result is very important and was unforeseen. If asis usually thecase, the reduction potential was taken as that on theplatinum-electrode, this reduction would be predicted at -'2,600 mV,therefore, before that of the negative active material (2,800 mV), andcharging (reduction) of the discharged negative active material would bedeemed impossible since electrolyte reduction should take place first.Therefore, it is due to the fact that the reduction potential of theelectrolyte is shifted on a lithium cathode towards a moreelectronegative value than that oxidation-reduction system LiCl/Li, thatcharging (reduction of oxided lithium) can be effected before thedecomposition of the electrolyte. This very favorable fact obtainedaccording to the invention is a consequence of the fact that lithium isin a metastable state with respect to the electrolyte.

In the same FIG. 2, the curve C relates to the anode polarization in thesame electrolyte.

FIG. 3 will now be referred to as illustrating an experimental deviceused according to the invention.

A lithium electrode is constituted by a disk 10 having a 6 cm area and a5 mm. thickness, prepared by punching, and then forced into a Teflon(polytetrafluoroethylene) mold 11. Such operations are efiected in gloveboxes under an argon atmosphere. A nickel wire 12 coated with Teflon 13is sharpened to a point at one end 14, and is laterally driven 1 cm deepinto the lithium disk 10 (FIG. 3). An excellent electrical Contact isthus obtained. The surface of the electrode is physically cleaned justbefore it is immersed in the electrolyte. The lithium chloride formed byoxidation is not adherent; in order to keep it upon the electrode, thelatter is placed horizontally and any stirring of the electrolyte isavoided. The cell is a glass one, provided with a metal cover andcontains about 150 ml of electrolyte. Dry argon is used as atmosphere.The platinum counter-electrode is placed in a compartment closed by asintered glass wall. The reference electrode AgCl/Ag and the lithiumelectrode are immersed in the same electrolyte.

The LiCl/Li electrode in a cell of this type then has been studied inconnection with two different kinds of electrolyte.

Using as a first electrolyte: propylene carbonate 0.5 mole/l NBut Cl(the result is shown in Curve D of FIG. 4).

Lithium is anodically oxided for a period of minutes at 0.5 A/cm in theelectrolyte. Current is then cut off and voltage is quickly stabilizedat a --2,740 mV (with 1 l0 mV accuracy), value as referred to theAgCl/Ag electrode.

This is in very good agreement with theory since the predicted value was2,830 mV.

Curve D of FIG. 4 shows the dependence of the current density from thelithium electrode voltage in the steady state. In other words, this isthe polarization curve of an insoluble electrode Li/LiCl in the selectedfirst electrolyte. A level anode current (corresponding to discharge) isobtained at 1.10 mA/cm this level being probably due to the Cliondiffusion. The current slowly decreases as time passes for the Cliondiffuses through a LiCl layer of increasing thickness. After 10 hours ofoxidation at the potential of the reference electrode the presence ofLiCl was observed by means of X- ray analysis. The cathode branch of thevoltage-current density curve (left part of the curve D corresponding tocharge) does not show any plateau and is limited by the solventreduction towards 3,400 mV as referred to the AgCl/Ag electrode.

Using as a second electrolyte, propylene carbonate +NJ But.,Cl+NBut,ClO(the results are shown in curves E, F and G of FIG. 4).

The oxidation of lithium still gives chloride since lithium perchlorateis soluble in propylene carbonate up to l mole/l. Polarization curves ofthe lithium electrode have been plotted for various values of both saltconcentrations. The results are shown in the following table III.

It may be seen that the presence of the perchlorate ion increases thelimiting anodic current density, i. e., the limiting discharge current.In the case of curves E and F, only LiCl was found by X-ray analysis.However, in the case of curve G a mixture of lithium chloride andperchlorate was found, lithium perchlorate being present as traces.

The meaning of curve D on the one hand, and curves E,F, and G on theother hand can be clarified, mainly as relates to anodic oxidation oflithium. These curves show the dependence of the current density uponthe steady state voltage, i. e. at a rate stabilized for a given period.But this period does not extend e. g. to the whole length of time ofelectrode discharge in the case of an anodic polarization correspondingto the right part of FIG. 4. The meaning of these curves is as follows:they define the trend of the system evolutions and they give acomparison of such evolutions. Thus it may be concluded from curve D onthe one hand and curves E, F and G on the other hand, that the dischargewithout polarization of the lithium electrode in the electrolyteconstituted by tetrabutylammonium chloride dissolved in propylenecarbonate, is clearly less advantageous than the discharge in anelectrolyte constituted by tetrabutylammonium chloride andtetrabutylammonium perchlorate dissolved in propylene carbonate. Theratio of the respective current densities was as about I to 12. Thus,the addition of tetrabutylammonium perchlorate may be seen to have avery significantly favorable effect upon the lithium electrode abilityto discharge without polarization. This actually constitutes animportant feature of the invention, allowing the obtention of highdischarge rates with electrodes of the insoluble type.

Referring now to table III, it may be seen that when the concentrationratio of NBut ClO to NBut Cl in moles per liter varies from 0.9/0.2=4.5to l/O.l=l0 going through 0.7/0.l=7, the value of the limiting anodiccurrent density does not vary much since it remains in the 13.8 to 11.8mA cm range. But it has been found that no lithium perchlorate was foundin the electrolyte for the 4.5 and 7 ratios whereas traces were foundfor the 10 ratio. As the lithium of this lithium perchlorate comes fromdissolved negative active material, it may be seen that in order toremain within the scope of the invention which relates to the insolubleoperation of the lithium electrode, the ratio of the concentrationsexpressed in moles per liter of tetrabutyl ammonium perchlorate andchloride in propylene carbonate should not exceed 10. This alsoconstitutes another important feature of the invention.

In FIG. 5, the constant current oxidation curves (i.e. the curves ofdischarge at constant current intensity) of lithium have been plottedfor electrolyte solution (1) of table III which gives higher anodiccurrent densities.

Curve I-I corresponds to a discharge at 5 mAlcm The curve begins at thesteady state voltage, i. e. about -2,750 mV as referred to the AgCl/Agelectrode. The discharge voltage decreases with time, which correspondsto an overvoltage of the lithium electrode. The maximum value of thisovervoltage corresponding to the trough of the discharge voltage isprobably due to a lag in crystallization which remained constant duringthe time of the test, this time being about 50 minutes. The plateaucorresponding to discharge is substantially linear at about 2,000 mV,which shows that no polarization perturbs the discharge.

Curve I corresponds to a 7 mA/cm discharge; it is similar to thepreceding one but the overvoltage slowly increases as time passes, whichmeans that there is a slight passivation or polarization.

Curve J corresponds to a 10 mA/cm discharge; it shows a steep increaseof overvoltage as time passes, which means that the electrode isstrongly polarized.

It may, therefore, be concluded that the lithium electrode associatedwith an electrolyte constituted by tetrabutylammonium perchlorate andchloride dissolved in propylene carbonate, though operating as aninsoluble electrode, can be discharged at a permanent rate, such as 6mA/cm without polarization, i. e., without passivation.

Due to this invention, the operation of the lithium electrode issatisfactory both for its substantial insolubility as for its ability tobe discharged at relatively high rates.

Thus, storage cells can be made with such electrodes, the said cellshaving a longer life than those of storage cells in which the negativeelectrode operates as a soluble electrode.

Another advantage is the simpler structure of the cell in which theseparator can be made of highly porous insulating materials such aspolyamide fiber felts, since there is no longer any risk ofshort-circuiting due to the fact that no lithium needles are formedduring charging. Thus, special semi-permeable membranes need not beused.

The invention is not limited to the described and illustratedembodiments which have been given as examples only. More particularly,the technical equivalents of the described means, and their combinationsare in the spirit of the invention and fall within the scope of theappended claims.

What is claimed is:

1. An electrochemical generator comprising a strongly electro-negativeelectrode of lithium, a positive electrode and a non-aqueous electrolytecomprising a mixed solution of tetrabutylammonium perchlorate andtetrabutyl ammonium chloride in propylene carbonate, said electrodeseach being reversible and insoluble in the said electrolyte.

2. An electrochemical generator according to claim 1, wherein theconcentration of the said solution in propylene carbonate is in therange of 0.5 to 1 mole per liter.

3. An electrochemical generator according to claim 1', wherein theconcentration ratio of tetrabutylammonium perchlorate totetrabutylammonium chloride expressed in moles per liter is about l0.

4. An electrochemical generator according to claim 1, wherein saidpositive electrode is of the Ag/AgCl type.

2. An electrochemical generator according to claim 1, wherein theconcentration of the said solution in propylene carbonate is in therange of 0.5 to 1 mole per liter.
 3. An electrochemical generatoraccording to claim 1, wherein the concentration ratio oftetrabutylammonium perchlorate to tetrabutylammonium chloride expressedin moles per liter is <about
 10. 4. An electrochemical generatoraccording to claim 1, wherein said positive electrode is of the Ag/AgCltype.
 5. A non-aqueous electrolyte for an electrochemical cell having astrongly electro-negative Li/LiCl electrode comprising a mixed solutionof tetrabutyl ammonium perchlorate and tetrabutyl ammonium chloride inpropylene carbonate.
 6. A non-aqueous electrolyte according to claim 5,wherein the concentration of mixed solution in propylene carbonate is inthe range of about 0.5 to 1 mole per liter.
 7. A non-aqueous electrolyteaccording to claim 5, wherein the concentration ratio of thetetrabutylammonium perchlorate to tetrabutylammonium chloride expressedin moles per liter is < about 10.